Vacuum tube oscillator



Feb. 5, 1946. -c. SHANK EIAL 2,394,018

' VACUUM TUBE QSCILLATOR Filed Abril 24, 1945 b I 1 18 I I :5 20 fi 12 1 POSITIVE 1E 13 24 TEMPERATUR 1 3 cosrn 1cm 1 e OF RESIST NCE ELEMENT 73 nsvmnou comsmzn ANODE AND CATHODE VOLTAGE msemv: TEMPERATURE I COEFFICIENT OF 7 z 30 -ooz RESISTANCE ELEMENT 2 B ---0.o4 I IL] 2 a- I I %DEVIATION compmin MODE AND CATHODE VOLTAGE INVENTORS CLARE M PIFt'NBEAG/l CLIFFORD ,4. 67/ IVA tube characteristics. proposed, for example, by vTerm'an et al., in the- Patented Feb. 5, 1946 VACUUM TUBE OSCILLATOR Cliiford A. Shank, Jackson Heights, and Clare M. Rifenbergh, Great Kills, Staten Island, N. Y., Iassignors to Federal Telephone and Radio Corporation, Newark, N. J a corporation of Dela ware Application April 24, 1943, Serial No. 484,794

4 Claims. (01. 250-36) This invention relates to improvements in vacuum tube oscillators, and more particularly to oscillators of the negative resistance type.

Vacuum tube oscillators of the negative resistance type have long been known forthe generation of relatively low frequencies, for example, audio frequencies up to about 20,000 cycles. It is also known to use a two-stage resistancecapacitance coupled amplifier of this type with positive and negative feedback together with a ballast resistor in the negative feedback circuit for the purpose of stabilizing the amplitude of the oscillations despite changes in input voltage, variable circuit parameters, supply voltage and Such an oscillator has been Proceedings of the I. R. E. (1939). However, none of these oscillators .are adapted for operation at high frequencies and tests have shown that when it is attempted to produce high frequency oscillations with these known circuits they are extremely unstable.

It is an object of our invention to provide an improved vacuum tube oscillator of the negative resistance type which is adapted to operate at much higher frequencies than comparable oscillators, while at the same time having optimum frequency stability characteristics.

A further object of this invention is the provision of a high frequency vacuum tube oscillator which is characterized by a high frequency stability despite wide variations in circuit parameters including fluctuations in anode and cathode supply voltages.

We attain these objects for the arrangement shown by way of example in the annexed drawing, wherein: I

Fig. 1 is a circuit diagram of a preferred form of high frequency vacuum tube oscillator according to the present invention;

Figs. 2 and 3 are graphs illustrating output voltage and frequency stability with respect to fluctuations in anode and cathode supply voltage according to our present invention; and

Fig. 4 is a partial circuit diagram illustrating a modified illustration of our invention.

In Fig, 1 we have illustrated a two-stage resistance coupled amplifier of generally known principles adapted, in accordance with the present invention to produce stable high frequency oscillations. The circuit includes a first vacuum tube l0 preferably of the pentode type coupled to a second amplifier tube l2, preferably of the tetrode type. The frequency at which the oscillator is adapted to work is determined by the constants of the tank circuit I4 feeding the control grid of the first tube I0 over the coupling condenser Regeneration is obtained by a first positive feedback circuit leading from the anode of the tube l2 back to the grid of the tube [0 through a condenser [6, while a second negative feedback circuit independent of the first feedback is connected from the anode of the tube [2 to the cathode of the tube ID. This second negative feedback circuit includes a main condenser l8 shunted by a variable padding con-' denser 20 by which the phase shiftin the negative feedback circuit can beproperly adjusted and the circuit asav whole correlated with-the constants of the tank circuit l4. The screen and suppressor grids of the pentode I0 and the screen grid of the tetrode l2 are connected in the usual manner. A device 22 such 'as a resistor having a positive temperature coefficient is connected'in the cathode circuit of the tube l0 and works in conjunction with the negative feedback circuit through condensers l8 and 20 in such'a manner that an increase in the amplitude of oscillations raises the current through the negative feedback circuit and through the device 22, resulting in an increase in resistance of the device 22 and thus increases the inverse feedback effect on the tube 12. Increase of feedback decreases the voltage amplification of the amplifier and thus tends to reduce the amplitude of oscillations. Thus, for example, if the cathode or anode voltage increases, tending to increase the amplitude of oscillations produced in the output circuit 24 of the tube l2, this undesired effect will be compensated for by a resultant decrease in amplification corresponding to the increase in feedback current and increase in the resistance of the device 22. Naturally, a decrease in cathode or anode voltages will have a reverse effect so that the final result is to produce an oscillator which is extremely stable. It will be noted, for example, that as compared with prior oscillators of a similar type, provision is made to correctly phase the feedback, an important consideration when operating at high frequency, while the positive and negative feedback circuits are independent of one another.

- Actual tests of the oscillator illustrated in Fig. 1, indicate that the results desired are obtained. For example at a frequency of 400 k. 0., the output voltage does not vary much more than 1.5% for combined anode and cathode voltage deviations up to plus or minus 30% as can be seen in Fig. 2. For negative voltage changes up to. minus 10% no measurable change in output voltage results, while positive voltage changes of over 20% result in only a 1% reduction in the output voltage.

The frequency stability is also extremely high. I

For a combined anode and cathode voltage deviation of minus 30%, the frequency will-only increase .02% while fora combined anode and cathode voltage deviation of plus 30%, the frequency deviation is only minus..02%.

deviations of anode and cathode voltage, the fre quency deviation is even less, as can be seen-fronr i a resistor or other device 26 having .a-negative temperature coefficient is serially inserted in the negative feedback circuit. The remaining ele ments of the circuit, including the tank circuit l4, thecondensers 16, I8 and 20, and the second amplifier, willremain the same as in the form of invention illustrated in Fig. l.

While we have described above the principles of our. invention in connection with certain specific circuits, it is to be clearly understoodthat this description is made only by Way of example and not as a limitation on the scope of our inventionas set forthin the. objectsand the ac.- companying claims;

We claim: Y

1. An oscillator including,.in combination, a tank circuit having apredetermined oscillating frequency, a first electron discharge :device having an anode, a cathode and'a grid, means for connecting one side of said tank circuit to the grid of-said electron discharge device, an amplifieriincluding a second electron discharge device For smallerhaving an anode, a cathode and a grid, an output circuit connected to said second amplifier, said output circuit including a lead common to the other side of said tank circuit and the cathode of said first electron discharge device, meanscoupling the anode of said first electron discharge device to: the grid of the second electron discharge device, a. first feedback circuit including a first condenser connected between the anode of said second electron discharge device and the grid of said first electron discharge device, a second feedback circuit independent of said first feedback circuit, including condenser means connected between the anode of said second electron discharge device and the cathode of said first electron discharge device, and resistor means having a predetermined temperature coefficient, connected to said cathode and responsive to changes in the amplitude of oscillations in said amplifier, for

2. The combination according to claim 1, in

V which .said last means comprises a resistor havfl ing a positive temperature coefflcient, connected between the junction of the second feedback circuit with the cathode of said first electrondischarge device and said common lead.

3.-The combination according to claim 1, in which said last means comprises a resistor having, a negative temperature coefiicient, serially connected said second feedback circuit.

4. The combination according to claim 1, in which said condenser means-includes afiXed condenser and a variable padder condenser connected in. shunt therewith'for correlatingsaid second-feedback circuit with said tank circuit.

CLIFFORD A, SHANK. CLARE M. RIFENBERGH. 

